1. Field of the Invention
This invention relates to an optical system, and particularly to an optical system including a curved surface (off-axial curved surface) on which with the route of a certain reference axis ray leading from an object surface to an image surface as a reference axis, a surface normal at a point on which the reference axis is incident does not coincide with the reference axis.
2. Related Background Art
As an optical system for forming the image of an object on an object surface onto an image surface, use has heretofore been made of a coaxial optical system in which on an optical axis, there is disposed a refracting surface or a reflecting surface rotation-symmetrical with respect to the optical axis.
Recently, however, in an image display apparatus such as a head mounting type display apparatus (HMD), an off-axial optical system using an asymmetrical aspherical surface not belonging to the category of the conventional coaxial optical system chiefly as a reflecting surface has come to be often seen. This is because the designing of such an optical system has become easy due to the improvements in the calculating capability of the computer and automatic designing technique.
In the above-described off-axial optical system, an off-axial reflecting surface or the like is used and therefore, the occurrence of eccentric aberration is unavoidable. Particularly, even in a light beam (on-axis light beam) relating to the center of the image field, astigmatism (on-axis astigmatism) occurs from the rotation asymmetry of the refractive power (power) of each surface.
In such an off-axial optical system, there is the condition that primary spherical aberration (on-axis astigmatism) can be corrected if all surfaces constituting the optical system are designed so as not to have azimuth dependency with respect to power.
However, if on an off-axial curved surface, the power thereof is designed so as not to have azimuth dependency, there arises the problem that the ratio of curvature in two cross-sections orthogonal to each other is determined and the degree of freedom of designing is taken away.